Structural health-monitoring is a challenging problem that is particularly difficult in large structures or in structures in which damage is not easily detectable. A variety of sensors (e.g., strain gages or accelerometers) are often needed to effectively identify structural damage. It is sometimes necessary to bond the sensors to the structure or embed them in the structure.
Effective health-monitoring of a composite structure is difficult because damage to the structure can occur during manufacturing or usage. The damage is not easily detected and decreases the integrity of the structure. Further, it is often undesirable or difficult to integrate discrete or distributed sensors into a composite structure. This makes it difficult to accurately identify structural damage. A need therefore exists for methods and apparatus that effectively identify damage to structures, and in particular, composite structures.
It is known in the art that certain materials exhibit a change in electrical resistance as a function of strain experienced by a material. A grid of members (e.g., constantan or copper traces) which resistance changes as a function of strain can be constructed and bonded to or integrated with a structural element (e.g., an aircraft wing) to detect the stresses experienced by the structural element. But, electrical connections must be made to each node of the grid. For large systems with many nodes, the sheer number of electrical connections becomes unwieldy as do the instrumentation required to measure the change in resistance of all the legs between the nodes.
U.S. Pat. No. 5,650,570, incorporated herein by this reference, discloses a sheet-like sensor with amorphous iron-based alloy members woven into glass cloth layers separated by an insulating sheet and covered by synthetic rubber sheets. The members of the first cloth layer run parallel to each other and the members of the second cloth layer run parallel to each other but perpendicular to the members of the first cloth layer. One end of all the members of the first cloth layer are electrically connected to a first scanner and the other end of all of the members of the first cloth layer are electrically connected to a first impedance analyzer. One end of all of the members of the second cloth layer are electrically connected to a second scanner and the other end of all of the members of the second cloth layer are electrically connected to a second impedance analyzer. In this way, the change in resistance along the length of any member due to strain can be measured and the strain computed.
Unfortunately, the specific location of the strain experienced by the sensor cannot be detected. The same is true if a member fails: the sensor cannot identify the specific location of a failure. Moreover, the maximum strain that can be computed is limited by the failure strain of the ferromagnetic elements used which is between 0.2% and 0.4%. Finally, the method disclosed in the '570 patent cannot accurately predict the stress distribution of a structural component since it only provides an estimate of where a force or pressure is applied.
A need therefore exists for methods and apparatus that effectively identify damage to structures.